Polyphenols in fruit and vegetables are thought to help reduce the risk of heart disease, cancer and other diseases.

The U.S. study was led by the chemist Dr Vinson from the University of Scranton in Pennsylvania. It is the first to establish that snack foods and some wholegrain breakfast cereals are a good source of polyphenols.

He said: 'We really were surprised by the levels of polyphenols we found in popcorn. I guess its because it's not processed. You get all the wonderful ingredients of the corn undiluted and protected by the skin. In my opinion it's a good health food.'

The study measured polyphenol concentration in breakfast cereals and snacks, showing there are more antioxidants in less processed wholegrain cereals, while popcorn has the most among the snack foods tested.

Hot breakfast foods such as porridge oats had disappointingly low levels of polyphenols, said Dr Vinson.